This invention relates to a process of forming a glass body from a vitrifiable batch and coloring, or modifying the color of, such body by causing a substance to diffuse into surface layers of the glass body from a contacting medium. The invention also relates to glass articles formed and colored by such process.
Colored glass bodies can be made by forming them from a vitrifiable batch incorporating appropriate coloring compounds. This procedure is practicable in but a limited number of circumstances due to the need to use a different batch composition for every different glass color to be produced. In the glass manufacturing industry and in particular in the production of flat glass it is usually much more practicable to color the glass body after its formation so that the coloring treatment can be controlled independently of the production of the vitrifiable batch and of the glass forming process.
A known process for coloring glass involves the diffusion of coloring substance into the glass at an elevated temperature. In this way the glass can be colored to a certain depth below its surface and the color cannot be removed simply by scratching the glass surface.
A coloring element which is of particular interest for various purposes is silver. Glass sheets having a yellow or yellow to brown coloration due to the presence of silver which has diffused into the glass can be used for example for glazing purposes to achieve a certain aesthetic effect or to screen off ultraviolet or short wave visible light.
The coloration of glass by silver diffusion is a process which has been found to be subject to haphazard, uncontrollable variations on account of which given results cannot be reproduced within required limits of accuracy without a considerable amount of expensive experimentation.
In a diffusion type coloration process the glass is contacted with a treatment medium providing coloring ions which diffuse into the glass.
It has been the belief that such process could be repetitively performed to color a plurality of glass bodies without causing inadmissible differences in coloration from one body to another by keeping the composition of the treatment medium reasonably constant. However, experiments with silver ion diffusion have shown that when a process is repeated after an interval of time using a treatment medium prepared to the same specifications as before, and observing the same temperature, treatment time and treatment procedure as on the first occasion, there is often a marked disparity in the results. This phenomenon remains unexplained.
Attempts to remedy this disparity by modifying the composition of the treatment medium or the treatment time or temperature afforded no satisfactory solution, the effects being too uncertain to make these expedients practicable. These circumstances have hitherto prevented the industrial application of a silver diffusion process where product quality control is important.